R.I. Tomov

University of Cambridge, Cambridge, ENG, United Kingdom

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Publications (29)42.98 Total impact

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    ABSTRACT: The objective of this study is to develop a cost-effective and environmentally friendly synthesis using soft chemistry based on Chemical Solution Deposition (CSD) using ink-jet printing for the deposition of ceramic thin films. The main advantages are the lower investment, the faster deposition with higher yield and the processing under ambient pressure enabling a complete continuous processing. The materials chosen for this study are TiO2 for development of photocatalytic layers; Yttria-stabilized zirconia (YSZ) is chosen as electrolyte material for Solid Oxide Fuel Cells (SOFC's) and for production of Thermal Barrier Coatings (TBC's); Lanthanum-Strontium-Manganese-Oxides (LSMO) magneto-resistive systems have been developed for obtaining functional magneto-resistive patterns.
    Nanoscience and Nanotechnology Letters 01/2013; 5(4):466-474. · 0.89 Impact Factor
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    ABSTRACT: A detailed procedure for the preparation of gadolinium doped (10 mol%) cerium (IV) oxide (CGO) suspension for inkjet printing is described in this paper. The optimisation of inkjet printing parameters for the deposition of solid oxide fuel cell electrolytes was also performed using a custom-built drop visualisation system. Additionally, the uniformity of the deposited drop relics on porous substrates was evaluated. The ink used in this study was an evaporative type comprising a solvent mixture of terpineol and methanol, ethyl cellulose and CGO powder. Successful printing of regular drops was achieved after printing optimisation. It has been demonstrated that inkjet printing is a promising technique for high quality membrane fabrication for applications including solid oxide fuel cells. The ink formulation and optimisation procedure would also be applicable for other ceramic ink development.
    Journal of the European Ceramic Society 08/2012; 32(10):2317–2324. · 2.36 Impact Factor
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    International journal of electrochemical science 01/2012; 7:6704-6721. · 3.73 Impact Factor
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    ABSTRACT: A sol–gel-based precursor solution of gadolinium-doped ceria (CGO) was developed for deposition by inkjet printing. A stable precursor was synthesised by dissolving stoichiometric amounts of cerium (III) acetate hydrate and gadolinium (III) acetate hydrate in propionic acid, and diluted to 0.75M with 1-propanol. The sintering behaviour of the printed precursor was investigated. Since the commonly used method of dilatometry is only applicable to bulk samples, an alternative approach using Differential scanning calorimetry (DSC) has been explored. The sintering temperature of the printed precursor was estimated by subtracting contributions from energy stored due to heat capacity and activation energy of ionic mobility from the DSC heat flow signal. Based on this modelling it was found that the optimum sintering temperature of the acetate-based CGO precursor was 1100±55°C, a result independently confirmed by SEM imaging of printed precursor coating on NiO-YSZ cermet. A gadolinium-doped ceria (CGO) thin film was then directly deposited on a porous NiO-YSZ cermet anode composite by inkjet printing. After co-sintering, it was shown that crack-free and continuous coating thinner than 10μm of CGO can be readily produced. These results suggest that the inkjet printing technique can be successfully implemented to fabricate a thin film of dense electrolyte (>98%) for solid oxide fuel cell (SOFC) applications.
    Journal of Materials Science 01/2011; 46(21):6889-6896. · 2.16 Impact Factor
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    ABSTRACT: Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO–YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO–YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm−2 at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.
    J POWER SOURCES. 01/2010; 195:7160-7167.
  • J SOL-GEL SCI TECHN. 01/2010; 54:154-164.
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    ABSTRACT: The successful inkjet printing of multiple cerium gadolinium oxide (Ce0.8Gd0.2O2) layers on highly textured Ni-5%W is reported using a stable ink, developing a solid-liquid interface comparable with that arising from dip coating. Two different approaches were used for the deposition of CGO layers using a 16-nozzle piezoelectric drop-on-demand print head. Two overlapping square arrays of droplets with constant volume and spacing were printed, with and without an intermediate CGO crystallization. The shortest possible heat treatment of the deposited layers was applied, potentially suitable for continuous large scale production. The results from X-ray diffraction show that the single phase Ce0.8Gd0.2O2 was obtained in all cases, but only the approach with intermediate CGO crystallization can produce a highly textured buffer layer. Optical micrographs and atomic force microscopy (AFM) also indicate the good quality of deposited films after heat treatment.
    Journal of Physics Conference Series 01/2010; 234(2).
  • Rumen I. Tomov, Mark Blamire
    Plasma Processes and Polymers 02/2006; 3(2):241 - 247. · 3.73 Impact Factor
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    ABSTRACT: Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents. Doping of 2 at. % Ti4+ ions increased the dc resistivity by more than three orders of magnitude. In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude. Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant.
    Applied Physics Letters 01/2005; 86(6):062903-062903-3. · 3.79 Impact Factor
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    ABSTRACT: La0.7Ca0.3MnO3/YBa2Cu3O7−δ/La0.67Sr0.33MnO3 heterostructural devices with double coercivity have been fabricated. The superconducting critical current (Ic) and critical temperature in both parallel (P) and antiparallel (AP) magnetic configurations remained unchanged within our measurement limits. This observation is contrary to results obtained elsewhere using similar metallic systems. A pseudo spin-valve magnetoresistive (MR) characteristic was observed at bias current (Ibias)∼Ic at temperatures below the onset of superconductivity. The effect increased with decreasing temperature and Ibias and can be explained using the assumption of the electron spin-charge separation.
    Physics Letters A 01/2005; · 1.77 Impact Factor
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    ABSTRACT: A number of liquid phase epitaxy (LPE) related growth methods have been investigated. These hybrid-LPE processes enable high rate 'liquid assisted' growth of epitaxial YBa2Cu3O7 films without the many disadvantages of classical LPE. Growth occurs by diffusive transport of Y through a thin liquid flux layer. This layer may be pre-deposited onto the substrate by various means including vacuum and non-vacuum techniques, or deposited at the growth temperature. The composition of the liquid layer is maintained during film growth by feeding YBa2Cu3O7, or the separate components, either from the vapour or by a powder route. Growth rates up to 10 nm s−1 have been demonstrated. Deposition of c-axis oriented epitaxial YBa2Cu3O7 is reported on both seeded and non-seeded substrates; the process is tolerant of a high substrate mismatch. Films 1–2 µm thick with K and a critical current density Jc> 2 MA cm−2 have been grown on a range of single crystal substrates as well as on buffered textured metallic tapes. The mechanism of nucleation and growth from a thin liquid layer is described within the general theoretical framework of crystal growth. Particular features of the growth are the short time constant for equilibration of transients in the deposition conditions, the wide range of relative supersaturation spanned by the process, and dominance of interface kinetic effects compared to volume diffusion in the liquid flux.
    Superconductor Science and Technology 08/2004; 17(10):1215. · 2.76 Impact Factor
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    ABSTRACT: Liquid phase epitaxy (LPE) has a great potential in producing low-cost, high-current, coated conductors due to its fast growth rate in excess of 1 µm min−1 and the capability of growing thick films up to 10 µm without degrading the structural perfection or Jc. The main problem for LPE is the chemical reaction between the films and substrates at elevated growth temperatures. Former efforts have been focused on the reduction of growth temperatures. This has been proved to be unsuccessful due to the limited degree of temperature reduction; reducing the growth temperature also reduces the solubility of YBCO in the liquid, making the growth even more difficult. An alternative solution to this problem is to search for a special buffer which is particularly resistant to the attack of the high-temperature liquid. We have recently developed a new buffer, Nd2CuO4, which was very stable in the cuprate solution at temperatures around 950 °C and therefore extremely useful for LPE. Initial growth of YBCO on Nd2CuO4 buffered, surface oxidized Ni substrates showed a sharp superconducting transition at 90 K and transport Jc over 105 A cm−2 (77 K). Although YBCO could be grown on Nd2CuO4 by LPE without any other intermediate seed layer, growth of Nd2CuO4 on both sides of the NiO/Ni substrate with 100% coverage was not easy and had a low success rate. Complete coverage of Nd2CuO4 buffer on the NiO/Ni substrates was the critical step to avoid Ni contamination and achieve a high Jc.
    Superconductor Science and Technology 07/2004; 17(10):1144. · 2.76 Impact Factor
  • B S H Pang, R I Tomov, M G Blamire
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    ABSTRACT: Multilayered structures of LaxSr1−xMnO3 (LSMO), LaxCa1−xMnO3 (LCMO) and Y Ba2Cu3O7−δ (YBCO) were successfully grown by 'eclipse' pulsed laser ablation deposition (PLD) on (100) SrTiO3 and (100) NdGaO3 substrates. X-ray diffraction and scanning electron microscopy were used to investigate epitaxial growth, interface quality and structural disorder. The composition and the roughness of the films was characterized by energy dispersive x-rays (EDX) and atomic force microscopy (AFM) respectively. The results are discussed in terms of the optimization of PLD parameters and the establishment of its limits. Issues such as interdiffusion and particulate contamination reduction as well as utilizations of the 'eclipse' technique are addressed. The interaction between superconductivity and magnetism was investigated by varying the thicknesses of YBCO layers while the keeping the thicknesses of the ferromagnetic layers constant.
    Superconductor Science and Technology 02/2004; 17(4):624. · 2.76 Impact Factor
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    ABSTRACT: Numerical modeling of heating and current-sharing effects on I–V curves of Y1Ba2Cu3O7 coated conductors and MgB2 wires was performed using finite element method software. The simulations were made when the conductors carried an over-critical current in steady-state conditions in zero applied magnetic field. Power law current–voltage characteristics typical of the real materials were used. It was assumed that the Y1Ba2Cu3O7 coated conductors were immersed in liquid nitrogen and MgB2 wires in a liquid helium bath. It was found that a bare MgB2 wire starts to be unstable already at applied electric fields >1 μV/cm. Y1Ba2Cu3O7 coated conductor was found to be much more stable, up to applied electric fields ⩾104 μV/cm and it can carry an over-critical current ≈1.4Ic.
    Physica C Superconductivity 01/2004; 401:140-145. · 0.72 Impact Factor
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    ABSTRACT: High temperature oxidation of cube textured (100)Ni and (100)Ni-0.1wt%(Mo or Mn) was carried out in a wide temperature regime between 900 and 1400 °C in air. Special attention was paid to avoid pre-oxidation at lower temperatures, thus favouring nucleation controlled surface oxidation epitaxy (SOE) as opposed to SOE achieved by competitive grain growth. Consequently, cube textured epitaxial NiO layers, under 1 mum in thickness, have been grown on pure Ni in a much wider working window than previously published. Furthermore, thin cube textured NiO layers have been grown on Ni-0.1%Mo tape in a still wider working window of ~200 °C between 1150 and 1350 °C. Preliminary results on Ni-0.1wt%Mn show a similar trend. An in-plane orientation of
    Physica C Superconductivity 01/2004; 405:219-226. · 0.72 Impact Factor
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    ABSTRACT: In this investigation, we deposited multilayers of [(La0.67Sr0.33MnO3)d/(YBa2Cu3Oy)x], [(La0.7Ca0.3MnO3)4/(YBa2Cu3O7)x]180nm and [(La0.45Ca0.55MnO3 )4/(YBa2Cu3O7)x]180nm of varying individual layer thicknesses using the ‘eclipse’ pulsed laser deposition technique. Transport measurements were performed to obtain the films’ critical temperatures (Tc). We study the interaction between superconductivity and magnetism within these heterostructures, paying attention to the superconducting and ferromagnetic coherence lengths, ξF and ξS respectively, of the materials in the system, and analyzing how these parameters affect the suppression of Tc, eliminating stray field as a possible cause of Tc suppression. We compared our data to previous work to achieve a more comprehensive study of oxide F/S multilayers. We observe that the magnetic nature of the manganite layers do not have much influence on Tc suppression in the multilayers. We show that within certain limits, the thicknesses of both the superconducting and ferromagnetic layers individually affect the Tc of the multilayers. The critical thickness of YBCO in our multilayers was estimated to be ∼20nm.
    Physica C Superconductivity 01/2004; 415(3):118-124. · 0.72 Impact Factor
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    ABSTRACT: Results on the angular and the field dependence of the critical transport current density as well as on the temperature dependence of the irreversibility fields in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> coated conductors are presented. The films were deposited by pulsed laser deposition on (Y<sub>2</sub>O<sub>3</sub>/YSZ/CeO<sub>2</sub>) triple buffer structures. Either textured magnetic Ni-Fe alloys or recently developed nonmagnetic Ni-Cr-W alloys with good biaxial texture (8.4° and 9.4° for ω and φ scans) were used as substrates. The influence of artificial pinning centers, introduced by fast neutron irradiation, on the irreversible properties of the YBCO films is discussed.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: Hysteresis losses of YBCO coated conductors on two different textured metallic substrates - (NiFe and NiCrW)- were measured. The measurements were performed in a dc SQUID magnetometer at 5 K in applied magnetic fields up to 6 T. An YBCO layer of the sample on NiCrW substrate was cut to form 2 filaments and measured again in a perpendicular magnetic field as well as in a field at an angle of 45° with respect to the tape face. In a perpendicular magnetic field the hysteresis losses of the 2 filaments were a factor of about 0.6 lower than the losses of the original single filament. The influence on the magnetization of dividing a monocore tape into 2 and 4 filaments was numerically modeled using the critical state model. For full penetration and a perpendicular magnetic field the magnetization decreases proportionally with increasing number of filaments. Hysteresis losses in metallic substrates were substantially lower than the total losses of the composite tapes.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: Oxide layers for the preparation of YBa2Cu3O7−x coated conductors were grown on highly textured Ni–12wt%Cr tapes in pure oxygen using surface oxidation epitaxy at temperatures between 1000°C and 1300°C. Microstructural investigations revealed a layered oxide structure. The upper layer consists mainly of dense cube textured NiO. This is followed by a porous layer containing NiO and NiCr2O4 particles. A detailed texture analysis showed a cube-on-cube relationship of the NiCr2O4 spinel to the metal substrate. Untextured Cr2O3 particles in a nickel matrix were found in a third layer arising from internal oxidation of the alloy. A high surface roughness and mechanical instability of the oxide were observed, depending on oxidation temperature and film thickness. However, mechanically stable oxide layers have been prepared using an additional annealing step in a protective atmosphere. Additionally, mechanical polishing or a second buffer layer, which grows with a higher smoothness, may be applied to reduce the surface roughness for coated conductor applications.
    Journal of Physics D Applied Physics 04/2003; 36(9):1053. · 2.53 Impact Factor
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    ABSTRACT: Thick NiO films were grown in air, on biaxially textured (0 0 1) Ni and as-rolled Ni tapes, at temperatures from 1050 to 1350 °C. Ni diffusion through the NiO film mainly contributes to the growth since is much faster than oxygen diffusion and occurs by a vacancy diffusion mechanism in the lattice at high temperatures. Parabolic growth kinetics were found for both NiO film thickness and grain growth, and compared with the literature data. Competitive growth of (1 1 1) and (0 0 1) oriented grains establishes the final NiO orientation at temperatures below 1250 °C, while at higher temperatures leakage diffusion at/towards grain boundaries, grain coarsening and (1 1 0) oriented grains disrupt the (1 0 0) texture. Hence, development of epitaxy of NiO on textured Ni tapes was found to be largely due to growth kinetics depending on both, time and temperature. We report here a systematic study of the microstructure and kinetics of formation of textured NiO substrate for application as a buffer layer in coated conductor technology.
    Physica C Superconductivity 01/2003; 385(3):337-345. · 0.72 Impact Factor